The present invention relates to a hammer assembly for a serial typing device for typewriters, text editing systems accounting machines, teleprinters and similar office machines, comprising a characters bearing element having flexible laminae which are selectively positioned in front of a platen; the hammer assembly comprises a printing hammer selectively movable for bringing a selected character towards the platen and an electromagnet which actuates the printing hammer. Typically, the character bearing element is of the "daisy" type, wherein the flexible laminae are radially mounted on a central hub, and wherein at the periphery of each lamina a particular character is mounted; the rotation of the hub brings the selected character in a printing position in front of the platen.
In devices of this type, the hammer is required to be operated in very high speed printing cycles and to have a long stroke, for effecting the typing of selected character. The long stroke is required for providing to the hammer a rest position far enough from the laminae, to permit the free rotation of the disc, and a work position thereof causing the selected lamina to flex to strike against the platen. Moreover, it is further required that the impact force of the hammer be varied in dependence either on the dimension of the selected character, or in dependence on the number of copies to be typed. The hammer accelerates during its strokes and its impact force against the platen is determined by the kinetic energy accumulated during its throw.
An assembly of the above-mentioned kind is known in which the typing hammer is actuated by an electromagnet of flat-faced armature type, wherein a short air gap is interposed between the armature and the pole pieces of the magnetic core. A lever connects the armature to the hammer for effecting the necessary long stroke of the hammer towards the printing position. The resulting assembly is cumbersome and costly. Moreover, the force applied by the armature to the hammer varies inversely with the gap length and complex and expensive electric circuits are necessary for obtaining the required variations of the intensity of the impact forces.
There are also known electromagnets having armatures of the plunger type which move through relatively long strokes and are subject to magnetic forces substantially constant during their movement. The forces induced on the armatures are thereafter directly used for actuating driven parts of machines. These electromagnets have been generally used on servomechanisms and the like, because of the simplicity in provision of the force versus displacement of these armatures and of their smoothness in operation.